1. Field of the Invention
This invention relates to electromagnetic flaw detection techniques and systems for use on metallic workpieces. Specifically, it relates to circuitry for balancing out unwanted error signals produced by flaw sensing elements of such systems.
2. Description of the Prior Art
Various methods and apparatus have been proposed for enabling the nondestructive testing of elongated metal workpieces such as wires, rods, tubing, pipes and billets. These proposals generally enabled detection of seams, voids and other defects which could be troublesome in a final product made from such workpieces.
In the past, a common method of inspecting such workpieces for defects was by visual observation. In spite of the utmost care, a mill inspector often overlooked seams or other defects. Moreover, visual inspection did not dependably determine the depth of a defect. Another problem with visual inspection is that it is dependent on human judgement and vision, both of which are subject to change, even in the case of the same inspector.
These problems have been overcome to a considerable extent by the use of automatic nondestructive testing equipment. Generally, such equipment operates by producing an electromagnetic exciting field in the region of the workpiece, and by moving a workpiece along a path through the field relative to a sensing element, such as a detection coil, also positioned in the field.
Such systems produced at the terminals of the coil a signal in response to nonuniformities in the electromagnetic field, which nonuniformities were in turn caused by inhomogeneities in the workpiece material passing by the coil.
Detection coils have been provided such that they either encircle the workpiece path of movement, or are placed adjacent that path. The signals produced by the detection coil actuated other apparatus which signalled the need for corrective action with respect to the flawed workpiece.
Where ferromagnetic workpieces are tested, it is known to provide a saturation circuit having a saturation coil connected to a source of D.C. voltage. The saturation coil induces a large D.C. magnetic field in the workpiece. The purpose of this saturation field is to reduce the magnetic permeability of the workpiece to near unity, to enable currents generated in the workpiece by the exciting field to penetrate the workpiece surface, and also to cancel out magnetic variations in the workpieces which might affect test results.
In eddy current testing, it is also known to use a separate exciter circuit including an exciter coil for inducing an alternating electromagnetic field in the workpiece. The exciter coil has either encircled, or been placed adjacent to, the workpiece path. In systems with a separate exciter circuit, the detection circuit has a separate detection coil also positioned near the workpiece path. The detector is connected to other circuit components for generating flaw indicating signals in response to voltage generated in the detection coil.
The flaw sensing coil has frequently been of the differential type. A differential coil consists of two series connected windings, substantially identical but wound with opposing magnetic polarity, longitudinally displaced and aligned along a common axis. The advantage of the differential coil is that it is not sensitive to even large magnitude electromagnetic fields, when those magnetic fields are substantially uniform over the entire region occupied by the coil. The differential coil produces no output in response to even large ambient steady state electromagnetic fields passing through it, such as from the exciter circuit. Rather, the differential coil responds only to localized nonuniformity in the field, such as results from the presence of workpiece flaws proximate the differential coil, when the flaws are geometrically "off center" with respect to the coil's length.
Another advantage of the differential coil is that a relatively small flaw, introducing a relatively small nonuniformity of the field induced in the workpiece, produces an output voltage across the differential coil which is more easily detected than that produced by a coil wound only in one direction. The coil wound only in one direction, when excited by an A.C. field, will constantly produce an output which is of relatively large magnitude. The presence of a small flaw in a workpiece moving with respect to such a detection coil alters the steady state signal by only a fractional amount. It is more difficult to detect small changes in a relatively large signal than to detect merely the presence of a signal, even though that signal may be small, where there was no signal before.
A problem with the use of differential coils is that most differential coils, however carefully constructed, are slightly unbalanced. They are unbalanced because the two oppositely wound portions frequently do not have identical configurations. In the presence of a uniform field, even a minutely unbalanced differential coil will produce a small but undesirable error output. The error output is undesirable in instances such as flaw testing because it is desired that the coil produce no output at all in the presence of an unflawed workpiece. The error output can sometimes be mistaken for a sign of a flaw, even though the workpiece may be perfect.
In order to obtain a proper zero output from a differential coil in the presence of an unflawed workpiece, balancing circuits have been devised to compensate for the imbalance which can result when even an unflawed workpiece is placed proximate the coil. In preparing such a system for operation, the balancing circuit is adjusted to achieve a zero output with an unflawed workpiece in proximity to the coil.
It is known to provide balancing circuitry in which the error signal produced across the entire differential coil is combined with an independently derived reference signal. In one form, the combination of signals is by a summing circuit. The reference signal is derived by adjusting the phase of a signal, having frequency equal to that of the field and of the error signal, to be precisely opposite the phase of the error signal. Additionally, the amplitude of the reference signal is adjusted to be equal to that of the error signal.
The previously used balancing circuits for differential coils have been manually operated. Manually operable controls have been used to adjust separately the amplitude and phase angle of the balancing signal.
Such balancing circuitry incorporated into a prior art flaw detection system is illustrated and described in U.S. Pat. No. 3,916,301, issued Oct. 28, 1975 for MAGNETIC FLAW DETECTION APPARATUS by Vild et al, which is expressly incorporated by reference here.
During normal testing conditions the sensing coil error signal may shift, thus requiring frequent readjustment of the manual controls to rebalance the test system. The test system must be monitored and rebalanced as necessary by a test operator.
While it is sometimes possible to maintain effective balancing by the use of the previously described circuitry and a diligent operator, there are obvious disadvantages attendant on this technique. The operator's time required can be costly, and his diligence must be depended upon to maintain effective and accurate testing conditions. Even a diligent operator can sometimes make mistakes in the frequent readjustments which are necessary. All these factors can reduce testing accuracy and increase operating cost.
It is known, in eddy current flaw detection apparatus, to employ a form of automatic balancing in an eddy current flaw detection apparatus utilizing a bridge detection circuit including two detection coils. This proposal involves circuitry for generating a supplemental signal to maintain the bridge in balance when an unflawed workpiece is near the detection coils.
Two alternating mutually orthogonal signals are produced. The two signals are automatically amplitude controlled such that the vector sum of their outputs constitutes a signal which will balance the bridge.
The effective dynamic range of the above described bridge correction circuitry is considerably limited, substantially circumscribing its ability to cope with error signals of widely varying phase and amplitude.
Since error signals can vary considerably in amplitude and phase angle where a differential coil is used, it is desirable that an automatic balancing system have dynamic range which is as great as possible.
Another disadvantage of this prior art proposal stems from the fact that the supplementary signal is injected directly into the detection coils. Such a technique can introduce undesirable transients which can cause spurious flaw indications.